I agree with kar120c. I just received my C&S card last week and called Chase to inquire about it since I did not get the usual call this 800 number to activate card sticker.
They said the following:
1. Both chip and mag stripe work on card.
2. Card # does not change, so old card will continue to work (I will take it as a backup to the new card on my next trip to EU, just in case)
3. Call Chase with travel plans (countries and dates of travel) before traveling abroad. Having spent months overseas each year this is something that I have always done. But the next comment from the Chase CSR came as a surprise...
4. Especially for the UK as fraud is very high there. Your card will be blocked in the UK unless you have called and put UK on your travel list. She didn't say "might be blocked" she said "will be blocked".
Of course it sounds like the OP actually spoke to Chase while in the UK, so I would hope that Chase and/or the OP would have specifically checked that during their discussions, but perhaps it didn't occur to either of them to do so. If not, then I would highly recommend the OP call Chase back and verify that UK is on his travel list and that the dates for travel to the UK are correct, as it sounds like that may be what the issue is.
And FWIW, my Chase MR Premier Visa has worked flawlessly overseas for the past few years, ever since they got rid of the FX fees.